1. Field of the Invention
This invention relates to the signaling of unbusy and busy line information on a digital data link, with particular applicability to low bandwidth data links.
2. Description of the Prior Art
A prior art search was conducted and the following patent references were uncovered:
U.S. Pat. No. 4,132,864 detects if a telephone is on or off hook in the presence or absence of a ringing current by monitoring the percentage of time the voltage across the phone is positive. Our invention relates to a digital rather than an analog communications channel, and uses the strength of the background noise rather than the positive duty cycle to distinguish between the on and off hook conditions.
U.S. Pat. No. 4,199,664 detects if a telephone is on or off hook in the presence or absence of a ringing current by observing a phase shift introduced by the impedance difference of the phone in the on and off hook conditions. Our invention relates to a digital rather than an analog communications channel and uses the strength of the background noise rather than the phase shift to distinguish between the on and off hook conditions.